Acute vision is key for birds to survive

The eyes of a hawk, like this red-tailed hawk, are positioned near the front of the head to provide binocular vision, meaning each eye focuses at the same time as in human vision. Photo Credit: Kathy Adams Clark. Restricted use. less

The eyes of a hawk, like this red-tailed hawk, are positioned near the front of the head to provide binocular vision, meaning each eye focuses at the same time as in human vision. Photo Credit: Kathy Adams ... more

Photo: Kathy Adams Clark

Image 2 of 3

The eyes of a hawk, like this red-tailed hawk, are positioned near the front of the head to provide binocular vision, meaning each eye focuses at the same time as in human vision. Photo Credit: Kathy Adams Clark. Restricted use. less

The eyes of a hawk, like this red-tailed hawk, are positioned near the front of the head to provide binocular vision, meaning each eye focuses at the same time as in human vision. Photo Credit: Kathy Adams ... more

Photo: Kathy Adams Clark

Image 3 of 3

The eyes of songbirds, like this chipping sparrow, are positioned on the sides of the head to provide monocular vision, meaning each eye focuses independently.﻿

The eyes of songbirds, like this chipping sparrow, are positioned on the sides of the head to provide monocular vision, meaning each eye focuses independently.﻿

Photo: Kathy Adams Clark

Acute vision is key for birds to survive

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Backyard songbirds, like cardinals and chickadees, see the world with visual clarity surpassing human eyesight.

Acute bird vision derives from the retina's fovea - the center of sharpest vision - that's densely packed with about 400,000 photoreceptor cones per square millimeter compared to about 200,000 in humans.

Bird eyes, in proportion to head size, are also larger than human eyes and therefore enable bigger, sharper images than are available to humans.

Red, green, blue and ultraviolet cones in bird eyes provide vision across a wider wavelength spectrum than seen by human eyes. Ultraviolet vision offers birds enhanced ability to detect food, such as seeds, insects, fruits, and in the case of hawks, small mammals.

Hawk eyes can focus on distant objects with up to five-power vision, which enables a soaring hawk to spot a mouse scurrying across your lawn. By comparison, typical binoculars for bird-watching have eight- or 10-power magnification.

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Eye examination

Upper and lower eyelids on birds close over the eye but are not used for blinking.

A sort of blinking is done by the transparent nictitating membrane between the eye's cornea and outer eyelids that sweeps horizontally across the eye to lubricate and protect the cornea.

Owls eyes have an internal reflecting structure called the tapetum lucidum that enhances light reception at night.

Information on bird collisions can be found at fws.gov/migratorybirds/pdf/management/reducingbirdcollisionswithbuildings.pdf

Hawk eyes are positioned near the front of the head to provide binocular vision, meaning that both eyes focus at the same time, as in human vision. But a songbird's eyes are positioned on the sides of the head to provide monocular vision, meaning each eye focuses independently.

Hence, a robin cocks its head to the side to get a sharper view of a worm slithering on the ground, but its eyes also give the bird a wide view of the surroundings and of potential predators.

But if birds have such good vision, why do they fly into windows?

When songbirds see a window's mirrored reflection of trees or open sky, they fly toward the illusory view and hit the window pane. The mirrored trick played on birds is like the trick a magician plays on our eyes with a mirrored illusion - except the illusion kills birds.

Stationary structures are certainly visible to birds, like grackles and pigeons, that navigate around Houston's downtown skyscrapers. But those birds fly in daylight, whereas most migratory songbirds fly during nighttime.

Why migratory birds fatally slam into stationary structures is unclear. Perhaps it's due the randomness of looming structures in their flight paths. But illuminating transmission towers and buildings further endangers birds as evidenced by the nearly 400 migratory songbirds crashing to their deaths against a lighted high-rise in Galveston earlier this spring.

One theory holds that migratory birds orient themselves by the stars and apparently race toward lighted buildings for orientation, but instead can crash to their deaths.

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